The present invention relates generally to transfer cases for use in four-wheel drive motor vehicles. In particular, the present invention is directed to a three-speed powershift transfer case.
Due to increased consumer demand for four-wheel drive vehicles, a plethora of different power transfer systems are currently utilized for directing power (i.e., drive torque) to all four wheels of the vehicle. In many applications, a transfer case is used to transfer power from the powertrain to the front and rear drivelines. Moreover, many modern transfer cases are equipped with a two-speed gear reduction unit for establishing high-range and low-range drive modes to accommodate differing road conditions and off-road terrains. Typically, the high-range drive mode is a direct speed ratio for use during most normal driving conditions while the low-range drive mode is an underdrive ratio useful during low-speed off-road conditions.
The two-speed gear reduction unit used in most transfer cases is either a layshaft arrangement or a planetary gearset with one or more clutches that are selectively actuated for making the drive connections. As a convenience feature, many gear reduction units now include a system for permitting such xe2x80x9crangexe2x80x9d shifting to be accomplished without first stopping the vehicle. In some instances, synchronized dog clutches are used to permit xe2x80x9con-the-movexe2x80x9d shifting between the high-range and low-range drive modes. Examples of such synchronized range shift systems for transfer cases are disclosed in commonly-owned U.S. Pat. Nos. 5,346,442; 5,655,986; 5,702,321; 5,836,847; and 5,902,205. Alternatively, it is known to employ a powershift clutch arrangement in two-speed transfer cases for shifting on-the-move between the high-range and low-range drive modes. In this regard, U.S. Pat. Nos. 5,443,429; 5,688,202; and 5,700,222 disclose powershift-type range shift systems in transfer cases.
In most four-wheel drive vehicles equipped with a transfer case, the transmission output shaft is coupled to the transfer case input shaft. However, in some applications, an add-on gearbox, commonly referred to as a compounder assembly, is installed between the transmission output shaft and the transfer case input shaft for providing an additional gear ratio, such as an xe2x80x9coverdrivexe2x80x9d ratio. An example of an add-on overdrive transmission assembly is shown in commonly-owned U.S. Pat. No. 4,798,103. Due to the long leadtime and cost associated with developing new multi-speed transmissions, the use of a compounder assembly in conjunction with an existing transmission for providing an additional gear ratio has found commercial success. One design compromise, however, is the additional length required in the vehicle""s powertrain to accept installation of the compounder assembly.
From the foregoing, it is apparent that transfer cases equipped with two-speed gear reduction units have been successfully integrated into four-wheel drive vehicles. However, a need still exists to develop alternative gear reduction units that further advance the technology in the field of four-wheel drive transfer cases.
It is therefore an object of the present invention to provide a three-speed transfer case for use in four-wheel drive vehicles.
As a related object, the transfer case of the present invention includes a powershift clutch assembly in association with a planetary gearset for establishing three distinct speed ratio drive connections between an input shaft and an output shaft.
According to another object, the present invention integrates the function of an add-on compounder and a two-speed gear reduction unit into a transfer case for use in four-wheel drive vehicles. In this regard, the three-speed transfer case of the present invention is operable to establish a first drive connection, a second drive connection, and a third drive connection between the input shaft and the output shaft.
According to the present invention, the transfer case includes an input shaft, an output shaft, and a planetary gearset connected therebetween. The gearset includes a first sun gear, a second sun gear, a carrier coupled for rotation with the input shaft, meshed pairs of first and second pinions rotatably supported on the carrier with each first pinion meshed with the first sun gear and each second pinion meshed with the second sun gear, and a ring gear coupled for rotation with the output shaft and which is meshed with the first or second pinions. The transfer case further includes a powershift clutch assembly comprised of a first range clutch located between the carrier and the ring gear, a second range clutch located between the first sun gear and a stationary member, and a third range clutch located between the second sun gear and the stationary member. Actuation of the range clutches is controlled by a hydraulic control system for shifting each range clutch between actuated and non-actuated modes to establish the three different speed ratio drive connections. The three speed ratios include a direct drive speed ratio, an underdrive speed ratio and an overdrive speed ratio.
In accordance with an alternative embodiment, the planetary gearset includes a compound pinion gear rotatably supported on the carrier and having a first pinion meshed with the first sun gear and a second pinion meshed with the second sun gear and the ring gear. Actuation of the three range clutches establishes three speed ratios including a direct speed ratio and two different overdrive speed ratios.